Liquid injection moldable organopolysiloxane compositions are known and used. A problem with all such compositions is that the hardness, tensile strength, elongation, tear and cure rates are so interdependent among themselves and also with the viscosity of the uncured liquid precursor that it is difficult to improve one property without deleterious effects on the other properties. Additionally, the kinetics and thermochemistry of the liquid injection molding process and the compositions used therewith have been such that only small lightweight articles of manufacture could be made by the techniques of liquid injection molding because of the speed with which the liquid precursor cures once it has been injected into the mold.
Liquid injection molding organopolysiloxane compositions are typically provided as two components that are mixed immediately prior to use. Both components contain alkenyl polymers, fillers, and in some cases resins. The first component contains a platinum catalyst while the second component contains a hydride crosslinker and cure inhibitors. The two components are mixed immediately prior to use in the injection molding apparatus. In addition to providing a so-called formulation pot-life, the inhibitor must prevent curing of the curable composition until the mold is completely filled. Once the mold is completely filled the inhibitor must then allow for a rapid cure of the curable or polymerizable composition in order to ensure a short cycle life.
U.S. Pat. Nos. 3,884,866 and 3,957,713 describe high strength addition cured compositions suitable for low pressure liquid injection molding. These compositions comprise a first component containing a high viscosity vinyl end-stopped organopolysiloxane, a low viscosity vinyl containing organopolysiloxane, filler, and platinum catalyst which is cured by mixing with a second component containing a hydrogen silicone composition. This composition has a low durometer, ca 20-35 Shore A, and moreover it is difficult to increase the durometer or hardness without adversely affecting other properties.
U.S. Pat. No. 4,162,243 discloses compositions similar to the previously referenced compositions but they contain as the most important distinction, fumed silica that has been treated with tetramethyldivinyldisilazane in addition to hexamethyldisilazane (disclosed in the '866 patent). The compositions of the '243 patent cure to elastomers having high hardness with good retention of other properties including strength, elongation, and tear in addition to having a low viscosity in the uncured state.
U.S. Pat. No. 4,427,801 extends the teaching of the '243 patent by incorporating a MM.sup.vi Q resin in addition to the vinyl containing treated fumed silica. This produces elastomers having even a higher hardness and tear strength but has the disadvantage of higher compression set and lower Bashore resilience. Recently, U.S. Pat. No. 5,674,966 has further improved upon the MM.sup.vi Q resin technology by incorporating a high alkenyl content silicone resin selected from the group of resins having the formula: M.sub.y M.sup.vi.sub.z D.sub.a D.sup.vi.sub.b T.sub.c T.sup.vi.sub.d Q and M.sub.y M.sup.vi.sub.z D.sub.a D.sup.vi.sub.b T.sub.c T.sup.vi d; preferably described by the formula: M.sup.vi.sub.x Q resulting in cured rubbers having a faster cure rate, high durometer, high resiliency, better compression set, and improved heat age stability.
The manufacturing technique of liquid injection molding typically has been limited to small parts, usually materials weighing less than from about 5 to about 50 grams. Advances in technology are allowing liquid injection molded parts to become larger. Larger parts require larger molds. Larger molds require more time to fill the mold with resin and thus curing must be inhibited for longer times in order to allow the mold to fill before cure may be initiated.
Silicone liquid injection molding materials are pumpable blends of silicone oligomers and polymers typically possessing a viscosity ranging from 200,000 to 3,500,000 centipoise (cps) at 25.degree. C. As presently utilized, these materials consist of two components that are mixed in a 1:1 ratio which when catalyzed will cure (or cross-link) to a rubber or elastomer upon heating. The first or "A" component typically contains siloxane polymers, fillers, various additives, and catalyst. The second or "B" component typically also contains siloxane polymers, additives, and fillers but also contains hydrogen siloxanes and cure inhibitors. The physical properties of both the cured and uncured materials depends in a very large degree upon the compositions utilized to generate the "A" and "B" components.
Typical liquid injection molding compositions consisting of the "A" and "B" components together generally are made up of the following ingredients:
1) from 50 to 75 parts by weight of a vinyl stopped polydimethylsiloxane having a viscosity ranging anywhere from 10,000 to 100,000 centipoise and a vinyl content of ranging from approximately 0.05 to 0.15 weight percent; PA1 2) up to 10 parts by weight of a low molecular weight vinyl stopped vinyl on chain polyorganosiloxane having a viscosity ranging from 200 to 1500 centipoise and a vinyl content of approximately 1.5 weight percent; PA1 3) up to 10 parts by weight of a low molecular weight mono-vinyl stopped polyorganosiloxane having a viscosity ranging anywhere from 500 to 2,000 centipoise; PA1 4) from 20 to 30 parts by weight of a fumed or pyrogenic silica having a surface area ranging from 150 to 450 m.sup.2 /gm; PA1 5) from 2 to 50 wppm of Pt hydrosilylation catalyst; PA1 6) from 0.01 to 0.50 parts by weight of an inhibitor compound; and PA1 7) from 100 to 1,000 wppm of silyl hydrides. Additional components may include extending fillers, coloring agents, additives to impart increased performance with respect to certain physical properties such as oil resistance, heat aging, ultra-violet stability and the like. PA1 1) hydrosilylation, PA1 2) free radical initiation, and PA1 3) high energy radiation initiation. PA1 1) in-situ filler treatment, PA1 2) post-reaction catalyst inhibition, and PA1 3) additives. PA1 1) preparing a curable silicone elastomer and PA1 2) adding thereto a salt of a cationic organic nitrogen compound PA1 1) an alkenyl organopolysiloxane and PA1 2) a hydrogen containing silicon compound selected from the group consisting of hydrogen containing silanes and hydrogen containing organopolysiloxanes. PA1 1) a monovalent cation having the formula: PA1 2) cations of cyclo-aliphatic nitrogen heterocycle having the formula: ##STR1## PA1 3) cations of cyclo-aromatic nitrogen heterocycle compounds having the formula: ##STR2## PA1 4) a cation of a nitrogen compound containing a nitrogen double-bonded to a carbon atom.
One particularly desirable attribute of a cured liquid injection molded material is a cured rubber having a high durometer. To obtain a high durometer rubber, one typical solution to this problem is to add a large amount of a filler anywhere from 25 to 70% by weight of the final cured rubber or elastomer. Use of large quantities of filler in a moderately viscous fluid such as the polymers used to prepare the precursor mixtures results in a fluid having high levels of suspended solids that significantly increase the viscosity of the mixture. Fumed silica is routinely used as a reinforcing filler to achieve high durometer in the cured rubber, however, at weight percent levels above 25 weight percent fumed silica, the liquid injection molding compositions become un-pumpable, defeating the purpose of liquid injection molding. Consequently, extending fillers are added and these usually impart color to the finished product. While this is not an undesirable result for many applications, it is occasionally a drawback.
Another approach to achieving a high durometer is to increase the cross link density of the cured rubber. It should be noted that because of the presence of vinyl groups, peroxide cures are not necessarily prohibited. Such formulations require the separate presence of olefinic unsaturation and hydride terminated siloxane species and are catalyzed by noble metal catalysts. While this results in a high crosslink density for the cured rubber, the drawbacks associated with these formulations is that although the desired high durometer is achieved the resulting cured rubbers suffer from very high moduli and very low elongations.
The properties of fabricated rubber depend not only on the chemical nature of the components but also on the properties of the filler, additives, and type of curing catalyst. Consequently, the resultant property profile of a given heat cured or liquid injection molded silicone rubber is highly dependent on the chemical nature of the various constituent components as well as the relative proportions of those components. For example, a high filler content increases hardness and solvent resistance of the resulting rubber. Such increased hardness and solvent resistance however, comes at the price of reduced physical properties such as elongation and tear, depending on the filler.
Not only do the properties of heat cured (i.e. high consistency or millable rubber) or liquid injection molded silicone rubbers vary with the nature of the silicone components and the various additives as well as their respective proportions but the properties also vary as a result of the various procedures used to compound the rubber. Properties of a heat cured rubber may therefore vary as a function of the thoroughness of the mixing and the degree of wetting of the filler by the component. Properties of liquid injection molded silicone rubbers will depend on the nature of the alkenyl and hydride components that form the basis of the elastomeric network, the catalysts employed, fillers and reaction inhibitors. All other factors being equal, a hydrophilic filler as opposed to a hydrophobic filler will impart significantly different properties to a finished rubber.
Further, properties of heat cured or liquid injection molded rubbers change with time. This is particularly true during the initial periods of the curing reaction. Since silicone rubbers are complex chemical mixtures, the cure reactions and associated side reactions never completely stop although they may slow down considerably after the initial cure. The properties of a heat cured or liquid injection molded rubber thus change slowly with age, preferably as slowly as possible.
Silicone rubbers may be cured by one of three general curing techniques:
For a hydrosilylation cure, high molecular weight polymers possessing a vinyl functionality are usually reacted with low molecular weight hydride-functional cross-linking agents. A stable platinum complex, functioning as a catalyst, is added along with an inhibitor to prevent cure initiation prior to heating. The reaction proceeds at room temperature, increasing reaction rate as the temperature is increased, to form a cross linked addition polymer.
Free radical curing of silicone rubbers is effected by heating the rubber precursor in the presence of a free radical initiator such as benzoyl peroxide. The predominant mechanism operating involves hydrogen abstraction from the methyl groups of the dimethylsiloxane moiety followed by radical attack on another methyl group creating a cross-linking ethylene bridge. If a small percentage of vinyl groups are present, the methyl radical can add to the vinylic double bond. In addition to benzoyl peroxide, other radical cure initiators include bis(2,4-dichlorobenzoyl)peroxide, tert-butyl peroxybenzoate, dicumyl peroxide, 2,5-dimethyl-di-(tert-butylperoxy)hexane, and 1,1-di-(tert-butylperoxy)-trimethylcyclohexane. Both 2,5-dimethyl-di-(tert-butylperoxy)hexane, and 1,1-di-(tert-butylperoxy)-trimethylcyclohexane are particularly useful and specific as free radical cure initiators for vinyl silicone heat cured rubbers.
High energy radiation, either as gamma rays or as an electron beam, can also effect cures. This type of cure causes a wide variety of bonds to be broken, thus cross-links occur between a variety of different atomic centers as the radicals created by the high energy recombine to form new chemical bonds.
When a heat cured or liquid injection molded rubber formulation is used to manufacture products such as gaskets, the particular end use and the environment of that end use govern how the material is formulated and processed. In the case of gaskets, compression set, sealing force, and retention of sealing force are important measures of performance. Compression set has been a significant factor in heat cured or liquid injection molded rubber technology for many years.
U.S. Pat. No. 2,803,619 discloses a polydimethylsiloxane gum filled with fumed silica and diatomaceous earth having a low compression set. The heat cured rubber of the '619 patent was cured by a peroxide initiated vulcanization lasting five minutes at 150.degree. C. followed by a twenty-four hour cure at 250.degree. C. The compression set was measured according to ASTM D-395 after being compressed to 75% of its original thickness for 22 hours at 150.degree. C. Subsequent to the '619 patent, post-bake curing times have been significantly reduced to conditions that avoid thermal decomposition of the silicone, e.g. 4 hours at 200.degree. C. as taught in U.S. Pat. No. 4,774,281. Such post-preparation finishing steps to control compression set add significantly to the cost of the materials.
Curing of a heat cured or liquid injection molded rubber begins when the cure is initiated during the molding process. The cure must be sufficiently rapid that the article can be removed from the mold without deformation. Yet the requirement that the finished product possess elastomeric properties in some degree means that the cure cannot proceed to the extent that the initially elastomeric heat cured or liquid injection molded rubber is no longer deformable. Thus the kinetics of the cure reaction must be carefully balanced for a rapid initial cure.
Subsequent developments have focused on three technical issues:
In-situ filler treatment may be divided into two broad classes: 1) hexamethyldisilazane and vinyl silazane treatment of the filler, and 2) hexamethyldisilazane and vinyl alkoxy silane treatments.
In the case of free-radical cures, generally peroxide initiated, the initiator is consumed. Use of gamma radiation or high energy electron beams also leaves no reactive residues in the rubber. When a hydrosilylation catalyst is used to effect a cure in a vinyl-hydride compound rubber, the cure must be controlled because the catalyst is not destroyed by the cure reaction. Thus a large variety of inhibitor compounds have been used: alkaline earth metal silicates (U.S. Pat. No. 3,817,910), metal sulfides (U.S. Pat. No. 5,219,922), boron compounds (U.S. Pat. No. 4,690,967), and various organic compounds (U.S. Pat. No. 5,153,244). Catalyst residues that remain in a heat cured or liquid injection molded rubber may continue to function catalytically leading to low levels of continuous cross linking reactions that deleteriously affect compression set.
Additives to heat cured or liquid injection molded rubbers to control compression set have most frequently involved the addition of substituted silicone resins. Recently, in sharp contrast, spinels have been used to control compression set (U.S. Pat. No. 5,260,364). Since the silicone resins added to the heat cured or liquid injection molded rubber formulation for compression set control are highly branched silicone resins, depending on when these resins are added can sometimes lead to the conclusion that these materials form part of the elastomeric matrix of the heat cured or liquid injection molded rubber.
A current problem not yet fully solved by the art deals with the incompletely reacted surface silanol groups of the various silica fillers currently in use. The presence of reactive, i.e. unreacted, surface hydroxyl or silanol groups in silica leads to condensation reactions and structuring of the silica component. One solution currently in use is to use silanol or methoxy stopped silicone fluids as blending agents to assist in dispersing the filler into the silicone component and also provide a reaction center that does not lead to structuring of the filler. In a sense, these blending agents are reactive diluents as they react with the filler surface hydroxyl or silanol groups preventing the condensation reactions between filler particles or filler and gum molecules that lead to stiffening and a loss of elastomeric properties.
U.S. Pat. No. 5,610,213 discloses control of the compression set of cold processable heat curable rubbers by means of controlling the density of unreacted surface hydroxyl groups of a fumed silica filler. Satisfactory values for the compression set are obtained when a fumed silica filler functioning as a reinforcing filler, having a BET surface area in the range of 90-400 m.sup.2 /gm has a residual level of surface hydroxyl groups determined by nitrogenous base chemisorption and magic angle spinning solid state nmr as being below a threshold value of 3.1 hydroxyl groups/nm.sup.2.
U.S. Pat. No. 5,569,688 ('688) discloses the use of ammonia generating additives to control the compression set of liquid injection molded or heat cured rubbers. Thus aqueous ammonia, urea and other compounds capable of generating ammonia by thermal decomposition as well as compounds that generate ammonia by chemical decomposition, such as hexamethyldisilazane are used to lower the compression set values for cured liquid injection molded silicone rubbers. U.S. Pat. No. 5,486,551 ('551) discloses the use of ammonium carbonate and ammonium formate as compression set additives, compounds which thermally degrade to liberate ammonia. In contrast to other art that recommends the use of fumed silica as a reinforcing filler in heat curable or liquid injection molded silicone rubber systems, both the '688 and the '551 use precipitated silica as a filler.